The present invention relates in general to loudspeaker systems and more particularly concerns a novel loudspeaker system characterized by unusually realistic reproduction of sound at relatively low cost in an attractive package that may be conveniently located in most listening areas. The present invention represents an attempt to achieve much of the performance level of the internationally famous BOSE DIRECT/REFLECTING loudspeaker system.
That system includes eight speakers on a pair of back panels that each form an angle of about 30.degree. with the wall upon which the rear speakers direct their sound and one speaker on the front panel that faces the room. This arrangement accomplishes two objectives. First, it radiates the desired ratio of about 8:1 reflected sound to direct sound. Secondly, by choosing the angles of the rear panel with the wall of about 30.degree., the BOSE DIRECT/REFLECTING 901 Loudspeaker system projects the image of a musical performance located on a stage that is about a foot behind the wall when the enclosure is about a foot in front of the wall. This arrangement establishes the musical source image to such an extent that it is possible to hear the full stereo spread from a wide range of listening positions including directly in front of one enclosure -- a feat that was not possible with conventional speakers.
The BOSE 901 DIRECT/REFLECTING loudspeaker system also embodies the concept, described in a paper entitled ON THE DESIGN, MEASUREMENT AND EVALUATION OF LOUDSPEAKERS by Dr. A. G. Bose presented at the 1968 Convention of the Audio Engineering Society, of the important advance of having a uniform radiated power response as a function of frequency. Stated in other words, if you put the loudspeaker system in its normal position and apply a signal of constant level to the system input over the audio frequency range, the acoustical power radiated as a function of frequency is substantially constant.
To achieve this result it was found necessary to establish the on-axis free field pressure response characteristic of the individual loudspeakers rising with increasing frequency. This will be better appreciated when it is recognized that at low frequencies the response of the system is essentially spherical. At higher frequencies the directional characteristic is more sharply beamed; therefore, uniform power response requires that the pressure response of the individual loudspeakers on axis rise with increasing frequency. Another reason for requiring this increase in response is that at low frequencies the individual loudspeakers radiate in phase, and the total power radiated is proportional to the square of the sum of the individual pressures. At higher frequencies the radiation from the individual loudspeakers is incoherent and the total power radiated is proportional to the sum of the squares of each of the individual pressure responses. The sum of the squares is less than the square of the sum.
The importance of this discovery can better be appreciated from considering the nature of a musical instrument. Consider a violin. It is apparent that the pressure radiation characteristic of such an irregularly shaped instrument as a function of frequency is anything but uniform. Yet, when one listens to a violinist playing, the general character of the sound does not change when the violinist plays with the violin oriented differently. Although the pressure response changes with direction, the total power radiated does not. These observed results tend to support the theory of the importance of having a uniform power response as a function of frequency because the loudspeaker will radiate the same relative power in each frequency band as the original instrument.
The BOSE 901 DIRECT/REFLECTING loudspeaker system achieves these and other advantageous results with an electronic active equalizer having more than a hundred components and transistors. The use of multiple full-range speakers helps eliminate audible resonances and avoids the disadvantages associated with woofers, tweeters, and crossovers. However, the use of so many loudspeakers and components in the equalizer is costly. Still another disadvantage of that system is that it must be spaced from the wall and floor to achieve most satisfactory performance.
Accordingly, it is an important object of this invention to achieve a number of the advantages associated with the BOSE 901 DIRECT/REFLECTING loudspeaker system at lower cost.
It is a more specific object of the invention to achieve the advantages of substantially uniform power response, satisfactory ratio of reflected-to-direct sound, remarkable realism, and full stereo spread from a wide range of listening positions with a loudspeaker system that may rest on the floor against the wall and be relatively inexpensive.